The long term goal is the elucidation of the atomic-level phenomena and mechanisms important to the understanding and ultimate modification of the biological occurrence, function and dysfunction of the apatite portion of dental hard tissue. The differences between dental "apatite" and stoichiometric hydroxyapatite are recognized as being characteristics with particular biological interest and the studies are therefore focussed on them. The principal emphasis in the currently proposed work is on (1) calcium and hydroxyl ion deficiencies (2) the role of chlorine and (3) the effects of minor substituents, all in human tooth enamel. In part (3), first Zr and then one or more of the elements Pb, Na, Mg, Zn, and Mn will be studied. The approach in all three parts involves (1) the determination of some substitution- interaction features in analog systems with fewer parameters, (2) the deduction of plausible models for the structural locations and roles of the ions and vacancies producing the differences from pure hydroxyapatite, and (3) the selection of the most probable models on the basis of the stringent requirement that they simultaneously account for all known physical and chemical characteristics of the natural material, tooth enamel. Principal tools are x-ray and neutron diffraction, infrared and laser-Raman spectroscopy, and various chemical analysis methods. Particular strength is given to the model assessment and iterative improvement by the use of the powerful new profile-fitting structure-refinement method applied to x-ray and neutron powder diffraction patterns of tooth enamel.